Driven equilibrium and fast-spin echo scanning

ABSTRACT

An NMR imaging process utilizes application of both a driven equilibrium technique and a fast-spin echo technique to acquire image. The fast-spin echo technique is a multiecho NMR imaging sequence, where different echoes are encoded differently to fill the (k x , k g ) space at a speed of 1/n of the single echo speed, where n is the number of echoes in the multiecho sequence. During this echo train, a 90-degree RF pulse applied with proper phase at the center of any echo turns the magnetization back in the direction of the static magnetic field. Within a short waiting time after the 90-degree RF pulse, the spins are ready to be excited again. The multi-echo sequence has one 90-degree RF pulse at the beginning, followed by a series of n 180-degree RF pulses, followed by n echoes. A second 90-degree RF pulse is turned on exactly at the center of the nth echo, which returns all the magnetization left at this time to the static field direction. Only one frequency is used for excitation in acquiring the NMR signal in the single slice mode. Gradients are adjusted for oblique scanning. In the multislice acquisition mode, RF phases of different slices are different from one another and the final images can be constructed either by sharing the k g  space or by using a transform process to separate the slices.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation of U.S. patent application Ser. No. 09/769,446,which was filed on Jan. 26, 2001 now abandoned.

FIELD OF THE INVENTION

In general, the present invention relates to methods of performingnuclear magnetic resonance (NMR) imaging. In particular, the presentinvention is a method of reducing the scan time in performing NMRimaging by utilizing driven equilibrium and fast-spin echo techniques.

BACKGROUND OF THE INVENTION

Magnetic resonance imaging (MRI) is one of the most versatile andfastest growing modalities in medical imaging. Since the discovery byDr. Raymond Damadian in the early 1970s that nuclear magnetic resonancetechniques can be used to scan the human body to yield useful diagnosticinformation, medical NMR imaging devices have been developed forobtaining NMR images of the internal structures of patients.Subsequently, much effort has been expended to improve and refine thetechniques used for obtaining NMR images, as well as to determine thediagnostic utility of NMR images. As a result, NMR imaging, or magneticresonance imaging, as it is sometimes known, has today proven to be anextremely useful tool in the medical community for the purposes ofdetecting and diagnosing abnormalities within the body.

Conventional magnetic resonance imaging techniques generally employpulsed magnetic field gradients to spatially encode NMR signals fromvarious portions of an anatomical region of interest. The pulsedmagnetic field gradients, together with radio frequency excitation ofthe nuclear spins and acquisition of signal information, are commonlyreferred to as a pulse sequence.

The basic science behind NMR imaging is as follows. Pulsed currentthrough a set of conductors will produce a magnetic field external tothe conductors; the magnetic field generally has the same time course ofdevelopment as the current in the conductors. The conductors may bedistributed in space to produce three orthogonal gradients X, Y, and Z.Each of the gradients may be independently pulsed by a separatetime-dependent current waveform.

In order to construct images from the collection of NMR signals,conventional NMR imaging equipment generally utilizes magnetic fieldgradients for selecting a particular slice or plane of the object to beimaged and for encoding spatial information into the NMR signals. Forexample, one conventional technique involves subjecting an object to acontinuous static homogenous field extending along a first direction,and to sets of sequences of orthogonal magnetic field gradients. Eachset of orthogonal magnetic field gradient sequences generates a magneticfield component in the same direction as the static field, but thesequences have strengths that vary along the direction of the gradients.

Generally, the NMR phenomenon occurs in atomic nuclei having an oddnumber of protons and/or neutrons. Due to the spins of the protons andneutrons, each such nucleus exhibits a magnetic moment. As a result,when a sample composed of such nuclei is placed in the homogeneousmagnetic field, a greater number of nuclear magnetic moments align withthe direction of the magnetic field to produce a net macroscopicmagnetization in the direction of the field. Under the influence of themagnetic field, the magnetic moments precess about the axis of the fieldat a frequency that is dependent upon the strength of the appliedmagnetic field and on the characteristics of the nuclei. The angularprecession frequency, ω, also referred to as the Larmor frequency, isgiven by the equation ω=γB whereγ is the gyro-magnetic ratio (which is aconstant for each particular atomic nucleus) and B is the strength ofthe magnetic field acting upon the nuclear spins.

When a substance such as human tissue is subjected to a uniform magneticfield (polarizing field B₀), the individual magnetic: moments of thespins in the tissue attempt to align with this polarizing field, but therandomly oriented magnetic components in the perpendicular (transverse)plane (X-Y plane) cancel one another. If, however, the tissue is alsosubjected to a magnetic field (excitation field B₁) that is in the X-Yplane and that is at the Larmor frequency, the net aligned moment, M,may be rotated, or “tipped”, into the X-Y plane to produce a nettransverse magnetic moment M_(t). M_(t) rotates, or spins, in the X-Yplane at the Larmor frequency. The practical value of thus phenomenonresides in the signal that is emitted by the excited spins after theexcitation field B₁ is removed.

Thus, the orientation of the moment (also known as the magnetization) Mcan be perturbed by the application of a magnetic field oscillating atthe Larmor frequency, which has the effect of rotating the magnetizationaway from the direction of the static field. Typically, the oscillatingmagnetic field is applied in a direction orthogonal to the direction ofthe static magnetic field by means of a radio frequency (RF) pulsethrough coils connected to a radio frequency transmitting apparatus. Inessence, the net magnetic vector or orientation of magnetization M isrotated away from the direction of the static field. One typical RFpulse is that which has either sufficient magnitude or duration torotate the magnetization M into a transverse plane (that is, 90 degreesfrom the direction of the static field) and thus is known as a 90-degreeRF pulse. Similarly, if the magnitude or duration of the RF pulse isselected to be twice that of a 90-degree pulse, the magnetization M willchange direction 180 degrees from the main or static magnetic field, andthe excitation pulse is called a 180-degree RF pulse.

Accordingly, a typical imaging procedure involves the use of threeorthogonal magnetic field gradients, X, Y, and Z, which are pulsedcoordinately along with bursts of radio frequency energy. For example,the Z gradient is pulsed on for two brief time periods. A 90-degreeradio frequency pulse in the first time period and a 180-degree radiofrequency pulse in the second time period are used to select a slice ofthe anatomy of interest, and to induce the nuclear spin system withinthat slice to generate an NMR signal. Once the slice is selected by theZ gradient, the two remaining orthogonal gradients are used to conferspatial encoding on the NMR signal in the two orthogonal directions. Forexample, the Y gradient will encode on the basis of phase advancesimparted on a series of :signal responses by using a pulsed gradientwaveform of progressively increasing area. The X gradient, which ispulsed on during the signal collection period, will frequency-encode theNMR signal in the third orthogonal direction.

When the RF excitation pulse is stopped (by turning the RF transmitteroff), the nuclear spins tend to slowly realign or relax back to theequilibrium position. At this time, the spins emit an NMR signal, whichcan be detected with an RF receiver coil (which may be, and often is,the same coil as that used with the transmitter). The emitted NMR signalis dependent on three basic parameters, namely, the density of theexcited nuclei, the spin-lattice (longitudinal) relaxation time (T1),and the spin-spin (transverse) relaxation time (T2). The latter twoparameters are both exponential time constants that characterize therate of return to equilibrium of the longitudinal and transversemagnetization components following the application of the perturbing RFpulse. These NMR: parameters of spin density, T1, and T2 are related tothe atomic nuclei subjected to the NMR phenomenon.

In accordance with this technique, nuclear spins in a selected plane areexcited by a selective RF pulse, in the presence of one of the magneticfield gradients. The frequency of the selective RF pulse corresponds tothe Larmor frequency for only the selected plane or the object asdetermined by the magnetic field gradient imposed on the static magneticfield. The applied magnetic field gradient is designated as the sliceselection gradient. The selected plane will therefore extend in adirection perpendicular to the gradient direction of the slice selectionmagnetic field gradient. The selected spins are then subjected to theother magnetic field gradients (which can be designated as the read outand phase-encoding magnetic field gradients). A plurality of repetitionsare utilized in which the amplitude of the phase:-encoding gradient isvaried for each repetition and in which the read out gradient is appliedduring the reading out of the generated NMR signals.

The NMR signal is processed to yield images that give an accuraterepresentation of the anatomical features in the selected slice, as wellas provide excellent soft tissue contrast. NMR signals may be processedusing various algorithms, depending upon the precise nature of the dataacquisition procedure. However, all methods employed rely on the abilityto spatially encode the signal information by making use of the magneticfield gradients, which are time modulated and sequentially pulsed invarious modes to effect the desired result.

For example, the received NMR signals may be transformed by utilizing,for example, conventional two-dimensional Fourier transform techniques.The read out magnetic field and phase-encoding magnetic field gradientsencode spatial information into the collection of NMR signals so thattwo-dimensional images of the NMR signals in the selected plane can beconstructed. During the scanning sequence, the various magnetic fieldgradients are repeatedly switched on and off at the desired intervals.

Many NMR imaging schemes rely on a collection of spin-echo NMR signals.In utilizing spin-echo signals, a 90-degree RF excitation pulse isfollowed by the application of a 180-degree rephasing RF pulse at apredetermined time interval after the 90-degree pulse. This produces aspin-echo signal at a corresponding time interval after the applicationof the 180-degree RF pulse. In NMR parlance, the time that the spin-echoNMR signal is produced after the 90-degree RF excitation pulse isdesignated as TE (for time of echo). Thus, the 180-degree RF pulse isapplied at a time interval of TE/2 after the 90-degree RF pulse.

In the application of NMR principles to medicine and medical diagnosticimaging of live human subjects, NMR signals are obtained for a multitudeof small areas in a patient, known as picture elements or pixels. Thepixels are used to construct an image or pictorial representation of aparticular area of the patient being examined. More particularly, theintensity of the NMR signals is measured for the multitude of pixels.The intensity of each signal is a complex function of the tissue-relatedparameters used in gathering the image information.

For example, it is known that variations in the relaxation times T1 andT2 are closely associated with the differences between healthy anddiseased tissue, and thus, from a diagnostic viewpoint, images thatdisplay or show significant T1 and/or T2 contrast have proven to be ofgreat diagnostic interest. Unfortunately, in conventional techniques forobtaining both T1-weighted data and T2-weighted data, not only areseparate scans required for obtaining T1-weighted and T2-weightedimages, but further, additional T1-weighted stains are required toobtain T1-weighted images for the same number of slices for whichT2-weighted scans are obtained. This increases the number of scans ofthe patient that must be performed and the time necessary to completesuch scans. After each imaging scan that is performed on a patient, itis generally necessary to allow the patient to rest. Also, a certainamount of time is necessary when conducting a scan for operator setup,loading information into the apparatus with respect to the conditionsand sequencing for collection of data, etc. Therefore, in order toobtain, using conventional techniques, T2-weighted images for aplurality of planes and a corresponding number of T1-weighted images,the total scanning time is quite long. It is apparent that one of themajor problems with medical NMR imaging is patient throughput. Thus,numerous efforts have been devoted to the development of techniques forobtaining images in a shorter period of time.

Although various efforts have been devoted to the development oftechniques for shortening the scan times, to date, they have generallyresulted in a sacrifice of the diagnostic quality of the informationobtained, and thus, have not yet proven satisfactory.

One factor contributing to the length of an imaging time period is theperiod of time required for the return of the nuclear magnetizations toequilibrium prior to the :subsequent excitation. A method that has beenused to shorten this time period is known as the driven equilibriumpulse technique. In typical driven equilibrium techniques utilized withspin echo sequences, spins in the X-Y plane are driven back to alignmentwith the Z-axis in order to shorten the time period required for thespins to return to equilibrium. As a result, the data acquisition timeis shortened, and image contrasts may be manipulated in pulse sequencesat a high repetition rate.

However, driven equilibrium techniques have not become a standard on aclinical scanner. A major problem with conventional driven equilibriumtechniques is the resulting lack of contrast control in the derivedimage. Some of the difficulty may be related to eddy current control andRF phase control in MRI scanners. That is, in order to obtain accurateimaging data using the driven equilibrium technique, it is importantthat all the spins in the selected slice are precisely in phase, andthat the 90-degree Z-restoring pulse is delivered along an axis that isexactly perpendicular to the direction along which the transversemagnetization is focused. Even small deviations in these parameterscaused by eddy currents generated by the gradients are enough toseriously degrade the amount of magnetization restored to the Z-axis.

Another technique for decreasing the scan and image capture time of anNMR imaging procedure is the use of fast-spin echo methods. Such methodsinvolve the acquisition of multiple spin echo signals from a singleexcitation pulse in which each acquired echo signal is separatelyphase-encoded. Each pulse sequence therefore results in the acquisitionof a plurality of views, and a plurality of pulse sequences is typicallyutilized to acquire a complete set of image data. However, using suchtechniques, although the TE time interval can be varied, the repetitionrate is that same for each produced image, thereby precluding thegeneration of truly T1-weighted images.

Accordingly, a significant need exists for shortening the time periodfor obtaining the T1-weighted and T2-weighted images, and in particular,to reduce the total acquisition time for acquiring the data andinformation from which T1-weighted and T2-weighted NMR images areconstructed.

SUMMARY OF THE INVENTION

It is therefore an objective of the present invention to provide areduced scanning and image data collection time for NMR imaging, withoutsacrificing image quality.

The present invention is an NMR imaging process. The process includessubjecting the imaging object to a uniform polarizing magnetic field,applying orthogonal magnetic field gradients to the imaging object,applying RF energy to the imaging object according to a fast-spin echotechnique, and subsequently applying RF energy to the imaging objectaccording to a driven equilibrium technique. According to this process,a nuclear magnetic resonance signal emitted by the imaging object may bedetected, and the nuclear magnetic resonance signal may be processed toprovide imaging data. Preferably, the fast-spin echo technique includesapplication of a multi-echo NMR imaging sequence. The multi-echo NMRimaging sequence includes a plurality of different echoes, and each ofthe plurality of different echoes is encoded differently, or at leastone echo is encoded differently than another echo. A 90-degree RF pulseis applied at the center of any of the plurality of different echoes.The applied 90-degree RF pulse has a phase such that magnetization ofthe imaging object is oriented in the direction of the uniformpolarizing magnetic field.

According to another aspect of the present invention, an NMR imagingprocess includes subjecting the imaging object to a uniform polarizingmagnetic field and applying orthogonal magnetic field gradients to theimaging object. A first 90-degree RF excitation pulse is applied,followed by a sequence of 180-degree RF excitation pulses, which in turnis followed by a second 90-degree RF excitation pulse. The process mayalso further include detecting a nuclear magnetic resonance signalemitted by the imaging object and processing the nuclear magneticresonance signal to provide imaging data. Each of the 180-degree RFexcitation pulses in the sequence generates a spin echo, and each spinecho precedes a next 180-degree RF excitation pulse in the sequence.Preferably, the second 90-degree RF excitation pulse is applied at acenter of the spin echo generated by a last 180-degree RF excitationpulse in the sequence. Each spin echo may be encoded differently, or atleast one spin echo is encoded differently than another spin echo. Thesecond 90-degree; RF excitation pulse has a phase such thatmagnetization of the imaging object is forced in the direction of theuniform polarizing magnetic field.

Thus, the multi-echo NMR imaging sequence preferably includes a first90-degree RF pulse followed by a series of 180-degree RF pulses. Theseries of 180-degree RF pulses includes n 180-degree pulses, which arefollowed by n echoes. A second 90-degree RF pulse is applied at a centerof the nth echo, such that magnetization of the imaging object isoriented in the direction of the uniform polarizing magnetic field.According to the described method, the fast-spin echo technique providesfor acquisition of multiple spin echo signals from a single excitationpulse, and spins are driven back to alignment with the polarizing field.Accordingly, image data is acquired more rapidly during a scan, and thetime period between scans is reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be more clearly understood by way of thefollowing written description, appended claims, and drawings, in which:

FIG. 1 a shows the timed application of RF and gradient pulses in aconventional/standard spin echo MRI scheme;

FIG. 1 b shows the timed application of a conventional drivenequilibrium MRI scheme;

FIG. 2 shows the conventional fast-spin echo MRI scheme; and

FIG. 3 shows a fast-spin echo imaging pulse sequence applied with drivenequilibrium pulses according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

As noted above, in a conventional spin echo imaging process, data iscollected following the 180-degree RF pulse. Prior to each acquisition,the phase-encoding gradient is stepped according to the conventionalspin warp imaging technique. The imaging data corresponding to each stepof phase-encoding gradient represents a line in the k-space (k_(x),k_(y)). Once the k-space is filled, a two-dimensional Fourier transformwould provide the necessary information to reconstruct an image. Asingle spin echo pulse sequence is shown in FIG. 1 a. As shown, thesequence includes X, Y, and Z gradients 101, 102, 103, and 90-degree and180-degree RF pulses 104, 105. The single echo sequence can be expandedto a multi-echo sequence by adding a number of subsequent 180-degreepulses. The time gap between the 180-degree pulses is twice that betweenthe 90-degree pulse and the first 180-degree pulse. This train of180-degree pulses produces echoes at the center of those RF pulses. TheT2 dependence of these echoes can be used to produce a number of imageshaving different contrasts.

A conventional driven equilibrium imaging pulse sequence is shown inFIG. 1 b. As shown, the sequence includes X, Y, and Z gradients 106,107, 108, 90-degree RF pulses 109, 112, and 180-degree RF pulses 110,111. All the details related to this technique have been described, forexample, by van Uijen and den Boef, “Driven-Equilibrium RadiofrequencyPulses in NMR Imaging,” Magnetic Resonance in Medicine 1, 502–507(1984), which description is incorporated herein by reference. Eventhough there are two 180-degree pulses 110, 111, only the echo followingthe first 180-degree pulse 110 is used for image reconstruction. Asecond spin echo is refocused exactly at the center of the second90-degree pulse 112. This 90-degree RF pulse 112, applied with properphase, turns the magnetization to the equilibrium direction.

A fast-spin echo sequence is shown in FIG. 2. As :shown, the sequenceincludes X, Y, and Z gradients 201, 202, 203, a 90-degree RF pulse 204,and a series of 180-degree RF pulses 205. Thus, this pulse sequence hasmultiple 180-degree refocusing RF pulses, which each generate an echobetween adjacent 180-degree pulses. Each of these multiple echoes isthen phase- encoded differently, so that the data from each echo fills adifferent line in k_(x), k_(y)-space. FIG. 2 shows one exemplary set ofphase-encoded levels used in scanning. In this example, there arefifteen 180-degree RF pulses 205 and fifteen echoes following these180-degree RF pulses. The eighth echo has zero for its phase-encodingamplitude (Y gradient 202). During MRI scanning, each of thephase-encoding amplitudes is stepped so as to generate data from eachecho that fills consecutive lines in (k_(x), k_(y)-space. All the datafrom these echoes make up the entire k-space data needed to construct animage. Fast spin echo technique is very well described in MRI literatureand those skilled in the field of MRI scanning are well aware of itspotential. One such advantage is speed, that is, reduction of scan time.Since the entire image is reconstructed from data from all the echoes,the number of echoes during a waiting period determine the number ofphase encoded levels acquired in a single waiting period. The number ofwait time (TR) repetitions needed to fill k-space is therefore reducedby the number of echoes in the pulse sequence train. For example, for a256 phase-encoding level scan with a sixteen echo train, only sixteenTRs are needed to collect the data needed to fill a 256×256 k-space.Therefore, the scan time is reduced by a factor equal to the number ofechoes in a TR. In a multi-slice scan, the slices are interleaved andthe pulse sequence is repeated a number of times that is equal to thenumber of slices.

According to the present invention, an imaging process utilizingapplication of both the driven equilibrium technique and a fast- spinecho technique can be used to acquire images faster than is possibleusing either of the two techniques alone. This process can be used inboth single-slice and multiple-slice modes. In the single-slice mode,the slices can be obliqued individually. The speed provided by thisprocess is particularly advantageous because a patient does not have tolie still in the scanner as long as with scanners using conventionalprocesses.

An exemplary driven equilibrium fast-spin echo pulse sequence is shownin FIG. 3. As in the previously described sequence, this sequenceincludes X, Y, and Z gradients 301, 302, 303, a 90-degree RIP pulse 304,and a series of 180-degree RF pulses 305. A second 90-degree RF pulse306 with proper RF phase centered on the last spin echo returns the leftover magnetization, determined by the signal strength at the center ofthe last echo, to the initial equilibrium direction parallel to thebackground static magnetic field direction. The magnetization requiresmuch less time to recover to its full equilibrium strength compared to astandard recovery time, which is usually 5 times the T1 value for agiven type of tissue.

In conventional fast-spin echo imaging, the contrast is controlled bycollecting the central phase-encoding levels (phase-encoding gradientamplitude close to zero) on the echo of choice. For example, the earlierechoes have greater signal from both short and long T1. However, byadjusting TR, one can control the type of image produced by collectingthe central phase-encoding levels on the early echoes. For short TRs, T1weighted images are obtained, and TRs greater than 5T1 produce protondensity images. By keeping TRs long (>5T1) and collecting central levelson later and later echoes, greater and greater T2 contrast is produced.The signal-to-noise ratio also varies with TR and effective TE (time toecho on which central levels are collected). For example, T1 weightedfast-spin echo scans have a lower signal-to-noise ratio than protondensity images, and T2 weighted images have greater signal-to-noiseratios in longer T2 tissues than in shorter T2 tissues.

Driven equilibrium fast-spin echo, on the other hand, achieves the samesignal-to-noise ratio as scans having a TR equal to 5T1. Contrast in theimage can be controlled in the same way as it is for conventionalfast-spin echo. In addition, greater T1 weighting may be obtained byreturning the signal from earlier echoes to the equilibrium direction.While driven equilibrium fast-spin echo and single TR fast-spin echoscans may take approximately same time, the signal-to-noise ratio ismuch greater using driven equilibrium fast-spin echo than it is usingsingle TR fast-spin echo. This is because the later echoes in a singleTR fast-spin echo sequence have more noise than signal, and thereforereduce the overall signal-to-noise ratio in the image. This is also truefor 3D driven equilibrium fast-spin echo scans. The shorter scan timeand higher signal-to-noise ratio of driven equilibrium fast-spin echoscans is beneficial in scanning patients with greater anxiety andclaustrophobia and in region sensitive to motion, such as c-spine andabdominal areas. The fast single slice capability is particularly usefulwhen rotation or angulation of slice is necessary to capture the correctanatomy of choice. The speed of driven equilibrium fast-spin echo canachieve the same result with better signal-to-noise ratio in multiplesingle slice scans.

Driven equilibrium fast-spin echo, as a single slice technique, can bebeneficial in may ways in scanning different types of patients, inaddition to being faster than conventional fast-spin echo techniques.The high speed of single slice driven equilibrium fast-spin echo can becompared to single shot ordinary projection X-ray radiograph. Theradiographic images are produced immediately, slice by slice, in acooperative environment between the patient and the scanner operator.This mode of operation can significantly enhance the effectiveness ofthe scanner utility. It is particularly useful in stand up MRI systems,where patients have to stand on their feet without moving from thebeginning until the end of the scan. In a single slice drivenequilibrium fast-spin echo patient/operator cooperative environment,patients simply walk inside the scanner space, hold one or more handlebars, and stay still only during a single slice acquisition period. Thisminimizes patient loading time and time lost due to poor qualityscanning.

There are two ways of running the single slice mode. In the first modeof operation, the region of interest is placed in the center of themagnet and imaged using driven equilibrium fast-spin echo. After thatslice is imaged, the patient bed is moved to position the next region ofinterest in the center of the magnet, and that region is imaged asbefore. By following this procedure, the Larmor frequency remainsconstant, that is, it is not changed to excite different slices. Thismode of operation significantly reduces the stringent and harder toachieve specification requirements of a whole body scanner. Accordingly,this can reduce the cost of the MRI scanners themselves, and make itmore affordable for a larger percentage of the population to undergo MRIdiagnosis. This mode of operation could be called the CAT Scan mode ofoperation because of the similarity of operation of the two diagnosticmodalities. According to the present invention, use of this mode doesnot limit the operator to axial plane imaging only; coronal planeimaging, for example, may be obtained by tilting the bed and the gantry.In other applications, the main magnetic field can be suitablyreconfigured to image other planes.

In the second single-slice mode of operation, the conventional wholebody magnet space may be used either for volumetric driven equilibriumfast-spin echo or repeated single slice imaging with driven equilibriumfast-spin echo. In this case, the Larmor frequency is adjusted to matchthe slice select gradient and the position of the next region ofinterest. This may be repeated until all the required slices areacquired. In the volumetric or 3D acquisition mode, the conventionalstepping second phase-encoding gradient is used along the slice/slabselection direction. A 3D Fourier Transform (3DFT) is used toreconstruct the final images.

In both modes of operation, individual slices or slabs may be orientedto image the anatomy of interest. Imaging the multiple signal sliceswith their respective individual slice orientations is particularlyuseful in obtaining both T1 weighted and T2-weighted images with greaterspeed and accuracy.

Driven equilibrium fast-spin echo can also be used as a simultaneousmultiple slice imaging technique. In a conventional multi-slice imagingprocess, the waiting period between excitation and acquisition of aslice from one region of interest is used excite and acquire otherslices from different regions of interest. Using a driven equilibriumfast-spin echo technique, this waiting period is reduced significantly,leaving no room for other slices to be taken. Therefore, a simultaneousmulti-slice excitation and acquisition technique is more advantageoususing the process of the present invention. The conventionalsimultaneous multi slice acquisition technique is very well known tothose skilled in the art of MRI scanning. The speed and utility ofdriven equilibrium fast-spin echo extends the range of application ofthe simultaneous multi-slice acquisition technique, and makes it moreuseful for applications to which the conventional technique applies.

Only one frequency is used for excitation in acquiring the NMR signal inthe single slice mode. Gradients are adjusted for oblique scanning. Oncethat slice is completely acquired, the NMR frequency and the gradientare adjusted to acquire an image from a different region with a secondoblique angle. The greater speed of single slice acquisition isbeneficial to the patient, who can relax between slices.

In the multislice acquisition mode, RF phases of different slices aredifferent from one another and the final images can be constructedeither by sharing the Kg space or by using a third fast Fouriertransform to separate the slices. All the slices have the same obliqueangle.

Thus, in general, the fast-spin echo technique utilized is essentially amultiecho NMR imaging sequence, where different echoes are encodeddifferently to fill the (k_(x), k_(g)) space at a speed of 1/n of thesingle echo speed, where n is the number of echoes in the multiechosequence. During this echo train, a 90-degree RF pulse applied withproper phase at the center of any echo turns the magnetization back inthe direction of the static magnetic field. The amount of returnedmagnetization is a function of how late in the echo train the 90-degreeRF pulse is applied. Within a short waiting time after the 90-degree RFpulse, the spins are ready to be excited again. The substantialreduction in the waiting time TR results in faster scans, and in thesingle slice mode, the scans can be obliqued to fit the anatomy ofchoice.

To summarize, a general multi-echo sequence has one 90-degree RF pulsein the beginning, followed by a series of 180-degree RF pulses. If thereare n 180-degree pulses, then n echoes follow those 180-degree pulses. Asecond 90-degree RF pulse is turned on exactly at the center of the nthecho, which returns all the magnetization left at this time to thestatic field direction. All the gradients used in imaging have tosatisfy the standard integral condition up to the center or the second90-degree pulse. Careful adjustment of integral conditions minimizesloss of signal-to-noise ratio.

A driven equilibrium fast-spin echo technique therefore overcomes theshortcomings of both conventional fast-spin echo techniques and drivenequilibrium methods, resulting in greater signal-to-noise ratio,scanning speed, and contrast control. The process of the presentinvention provides both single slice and multiple slice capabilities,and allows for individual slice orientation and simultaneous multipleslice scanning. Proper utilization of the technique reduces demand onthe magnet, at least in part by reducing the required gradient power,resulting in less expensive scanner hardware and a more cost-effectivescanning process. The utility and efficiency of particular MRIprocedures, such as stand- up image scanning, are enhanced. While theseimprovements and advantages are certainly noteworthy, it is important torecognize that 3D Fourier transform spin echo scanning techniques areonly possible using the process of the present invention, and are noteven contemplated through the use of conventional spin-echo scanningprocesses.

The present invention has been described by way of example and in termsof preferred embodiments. However, it is to be understood that thepresent invention is not strictly limited to the particularly disclosedembodiments. To the contrary, various modifications, as well as similararrangements, are included within the spirit and scope of the presentinvention. The scope of the appended claims, therefore, should beaccorded the broadest possible interpretation so as to encompass allsuch modifications and similar arrangements.

1. A method of performing a magnetic resonance scan sequence to modifyimage contrast, comprising: choosing an imaging pulse sequence, whereinthe repetition time of said pulse sequence is variable, and setting avalue for the variable repetition time that provides a desired imagecontrast; selecting a number of image slices to cover an anatomicalregion of an imaging object to be scanned, wherein the selected numberof image slices is greater than the number of slices that can beprovided by the set pulse sequence repetition time; acquiring image datafor a subset of the selected number of image slices at the set pulsesequence repetition time, wherein the subset includes at least oneslice; and automatically acquiring data for another subset of theselected number of image slices at the set pulse sequence repetitiontime, wherein the another subset includes at least one slice.
 2. Themethod of claim 1, wherein the imaging object is a human being orientedsubstantially upright during the magnetic resonance scan sequence. 3.The method of claim 1, wherein the desired image contrast is an imagecontrast that falls within a predetermined desired range of imagecontrast values.
 4. The method of claim 1, wherein automaticallyacquiring data for another subset of the selected number of image slicesincludes sequentially acquiring data for a plurality of subsets of theselected number of image slices, such that the data acquired for all ofthe plurality of subsets includes data corresponding to all of theselected number of image slices.
 5. The method of claim 1, whereinacquiring image data includes subjecting the imaging object to a uniformpolarizing magnetic field, applying magnetic field gradients to theimaging object, applying RF energy to the imaging object according to afast-spin echo technique, and applying an imaging sequence that producesT1-weighted images.
 6. The method of claim 1, wherein acquiring imagedata includes subjecting the imaging object to a uniform polarizingmagnetic field, applying magnetic field gradients to the imaging object,and applying RF energy to the imaging object according to a fast spinecho technique, wherein the fast-spin echo technique includes applyingan imaging sequence that produces T1-weighted images.
 7. The method ofclaim 6, further comprising collecting central phase-encoding levels onearly echoes of the imaging sequence; wherein a repetition wait time ofthe fast-spin echo technique is less than about 5T1.
 8. The method ofclaim 1, wherein acquiring image data includes subjecting the imagingobject to a uniform polarizing magnetic field, applying magnetic fieldgradients to the imaging object, applying RF energy to the imagingobject according to a fast-spin echo technique, and applying an imagingsequence that produces T2-weighted images.
 9. The method of claim 1,wherein acquiring image data includes subjecting the imaging object to auniform polarizing magnetic field, applying magnetic field gradients tothe imaging object, and applying RF energy to the imaging objectaccording to a fast-spin echo technique, wherein the fast-spin echotechnique includes applying an imaging sequence that producesT2-weighted images.
 10. The method of claim 9, further comprisingcollecting central phase-encoding levels on late echoes of the imagingsequence; wherein a repetition wait time of the fast-spin echo techniqueis greater than about 5T1.
 11. The method of claim 1, wherein acquiringimage data includes subjecting the imaging object to a uniformpolarizing magnetic field, applying magnetic field gradients to theimaging object, applying RF energy to the imaging object according to afast-spin echo technique, and applying an imaging sequence that producesproton density images.
 12. The method of claim 1, wherein acquiringimage data includes subjecting the imaging object to a uniformpolarizing magnetic field, applying magnetic field gradients to theimaging object, and applying RF energy to the imaging object accordingto a fast-spin echo technique, wherein the fast-spin echo techniqueincludes applying an imaging sequence that produces proton densityimages.
 13. The method of claim 12, further comprising collectingcentral phase-encoding levels on early echoes of the imaging sequence;wherein a repetition wait time of the fast-spin echo technique isgreater then about 5T1.
 14. The method of claim 1, wherein acquiringimage data includes subjecting the imaging object to a uniformpolarizing magnetic field, applying magnetic field gradients to theimaging object, applying RF energy to the imaging object according to afast-spin echo technique, and subsequently applying RF energy to theimaging object according to a driven equilibrium technique.
 15. Themethod of claim 14, wherein the imaging object is a human being, and theuniform polarizing magnetic field is produced by a magnetic resonanceimaging system, a wherein the human being is oriented substantiallyupright within the uniform polarizing magnetic field.
 16. The method ofclaim 14, wherein the fast-spin echo technique includes applying animaging sequence that produces T2-weighted images.
 17. The method ofclaim 16, further comprising collecting central phase-encoding levels onlate echoes of the imaging sequence; wherein a repetition wait time ofthe fast-spin echo technique is greater than about 5T1.
 18. The methodof claim 14, wherein the fast-spin echo technique includes applying animaging sequence that produces proton density images.
 19. The method ofclaim 18, further comprising collecting central phase-encoding levels onearly echoes of the imaging sequence; wherein a repetition wait time ofthe fast-spin echo technique is greater than about 5T1.
 20. The methodof claim 14, wherein the fast-spin echo technique includes applying amulti-echo NMR imaging sequence.
 21. The method of claim 20, wherein themulti-echo NMR imaging sequence includes a plurality of differentechoes, and wherein each of the plurality of different echoes encodeddifferently.
 22. The method of claim 20, wherein the multi-echo NMRimaging sequence includes a plurality of different echoes, and whereinat least one of the plurality of different echoes is encoded differentlythan another one of the plurality of different echoes.
 23. The method ofclaim 20, further comprising applying a 90-degree RF pulse at the centerof any of the plurality of different echoes.
 24. The method of claim 23,wherein the applied 90-degree RF pulse has a phase such thatmagnetization of the imaging object is forced in the direction of theuniform polarizing magnetic field.
 25. The method of claim 20, whereinthe multi-echo NMR imaging sequence includes a first 90-degree RF pulsefollowed by a series of 180-degree RF pulses.
 26. The method of claim25, wherein the series of 180-degree RF pulses includes n 180-degreepulses, which are followed by n echoes.
 27. The method of claim 26,further comprising applying a second 90-degree RF pulse at a center ofthe nth echo, such that magnetization of the imaging object is orientedin the direction of the uniform polarizing magnetic field.
 28. Themethod of claim 1, wherein acquiring image data includes subjecting theimaging object to a uniform polarizing magnetic field, applying magneticfield gradients to the imaging object, applying a first 90-degree RFexcitation pulse, applying a sequence of 180-degree RF excitation pulsesfollowing the first 90-degree RF excitation pulse, and applying a second90-degree RF excitation pulse following the sequence of 180-degree RFexcitation pulses, wherein each said 180-degree RF excitation pulse inthe sequence generates a spin echo, and wherein at least one said spinecho is encoded differently than another said spin echo.
 29. The methodof claim 28, further comprising: detecting a nuclear magnetic resonancesignal emitted by the imaging object; and processing the nuclearmagnetic resonance signal to provide imaging data.
 30. The method ofclaim 29, wherein the first 90-degree RF excitation pulse corresponds tothe angular precession frequency for a selected plane of the imagingobject.
 31. The method of claim 30, further comprising, after providingthe imaging data, moving the imaging object and applying the first90-degree RF excitation pulse corresponding to the same angularprecession frequency, to select a different plane of the imaging object.32. The method of claim 30, further comprising, after providing theimaging data, applying the first 90-degree RF excitation pulsecorresponding to a different angular precession frequency, to select arespective different plane of the imaging object, without moving theimaging object.
 33. The method of claim 28, wherein each said spin echoprecedes a next 180-degree RF excitation pulse in the sequence.
 34. Themethod of claim 28, wherein the second 90-degree RF excitation pulse isapplied at a center of the spin echo generated by a last 180-degree RFexcitation pulse in the sequence.
 35. The method of claim 28, whereineach said spin echo is encoded differently.
 36. The method of claim 35,wherein each said spin echo is encoded by one or more of the magneticfield gradients; and the one or more magnetic field gradients thatencodes the spin echoes is stepped to amplitude to encode each said spinecho differently.
 37. The method of claim 36, wherein the one or moremagnetic field gradients that encodes the spin echoes is stepped inamplitude to generate data from each said spin echo to fill respectivedifferent lines in three-dimensional k-space.
 38. The method of claim37, wherein all the data generated from the spin echoes fills the entirethree-dimensional k-space so that an image of the imaging object can beconstructed from the data.
 39. The method of claim 37, wherein thedifferent lines in three-dimensional k-space are consecutive lines inthree-dimensional k-space.
 40. The method of claim 28, wherein thesecond 90-degree RF excitation pulse has a phase such that magnetizationof the imaging object is forced in the direction of the uniformpolarizing magnetic field.
 41. The method of claim 28, wherein theimaging object is a human being, and the uniform polarizing magneticfield is produced by a magnetic resonance imaging system, wherein thehuman being is oriented substantially upright within the uniformpolarizing magnetic field.
 42. The method of claim 1, wherein acquiringimage data includes subjecting the imaging object to a uniformpolarizing magnetic field, applying magnetic field gradients to theimaging object, applying RF energy to the imaging object according to afast-spin echo technique, subsequently applying RF energy to the imagingobject according to a driven equilibrium technique, detecting a nuclearmagnetic resonance signal emitted by the imaging object, and processingthe nuclear magnetic resonance signal to provide imaging data, whereinapplying RF energy to the imaging object according to a fast-spin echotechnique includes applying an RF pulse corresponding to the angularprecession frequency for a selected plane of the imaging object.
 43. Themethod of claim 42, further comprising, after providing the imagingdata, moving the imaging object and applying an RF pulse correspondingto the same angular precession frequency, to select a different plane ofthe imaging object.
 44. The method of claim 42, other comprising, afterproviding the imaging data, applying an RF pulse corresponding to adifferent angular precession frequency, to select a respective differentplane of the imaging object, without moving the imaging object.
 45. Amagnetic resonance scan system, comprising: an imaging pulse sequencegenerator that generates a pulse sequence having a repetition time, thatis variable and settable by a user to a value that provides a desiredimage contrast; sets of conductors that carry electrical current andthat are distributed to produce orthogonal magnetic field gradients thatisolate image slices to cover an anatomical region of an imaging objectto be scanned; a controller that selects a number of image slices tocover the anatomical region of the imaging object to be scanned, whereinthe selected number of image slices is greater than the number of slicesthat can be provided by the pulse sequence repetition time set by theuser; and image data acquisition apparatus that acquires image data fora subset of the selected number of image slices as at the set pulsesequence repetition time, wherein the subset includes at least oneslice, and automatically acquires data for another subset of theselected number of image slices at the act pulse sequence repetitiontime, wherein the another subset includes at least one slice.
 46. Thesystem of claim 45, wherein, the desired image contrast is an imagecontrast that falls within a predetermined desired range of imagecontrast values.
 47. The system of claim 45, wherein the image dataacquisition apparatus is adapted to sequentially acquire data for aplurality of subsets of the selected number of image slices, such thatthe data acquired for all of the plurality of subsets includes datacorresponding to all of the selected number of image slices.
 48. Thesystem of claim 45, wherein the image data acquisition apparatus isadapted to subject the imaging object to a uniform polarizing magneticfield, apply RF energy to the imaging object according to a fast-spinecho technique, and subsequently applying RF energy to the imagingobject according to a driven equilibrium technique.
 49. The system ofclaim 48, wherein the uniform polarizing magnetic field is produced animaging space adapted for accommodating a human being oriented in asubstantially upright position.
 50. The system of claim 49, wherein theimaging space is adapted for accommodating a human being oriented in astanding position.
 51. The system of claim 48, wherein the fast-spinecho technique includes a multi-echo NMR imaging sequence.
 52. Thesystem of claim 48, wherein the fast-spin echo technique includes animaging sequence that produces T1-weighted images.
 53. The system ofclaim 52, wherein the image data acquisition apparatus is adapted tocollect central phase-encoding levels on early echoes of the imaging;sequence, and to set a repetition wait time of the fast-spin echotechnique to less than about 5T1.
 54. The system of claim 48, whereinthe fast-spin echo technique includes an imaging sequence that producesT2-weighted images.
 55. The system of claim 54, wherein the image dataacquisition apparatus is adapted to collect central phase-encodinglevels on late echoes of the imaging sequence, and to set a repetitionwait time of the fast-spin echo technique to greater than about 5T1. 56.The system of claim 48, wherein the fast-spin echo technique includes animaging sequence that produces proton density images.
 57. The system ofclaim 56, wherein the image data acquisition apparatus is adapt tocollect central phase-encoding levels on early echoes of the imaging;sequence, and to set a repetition wait time of the fast-spin echotechnique to greater than about 5T1.
 58. An NMR imaging process,comprising: subjecting the imaging object to a uniform polarizingmagnetic field; applying magnetic field gradients to the imaging object;applying RF energy to the imaging object according to a fast-spin echotechnique; subsequently applying RF energy to the imaging objectaccording to a driven equilibrium technique; and applying a 90-degree RFpulse at the center of any of the plurality of different echoes; whereinthe fast-spin echo technique includes application of a multi-echo NMRimaging sequence; and wherein the applied 90-degree RF pulse has a phasesuch that magnetization of the imaging object is forced in the directionof the uniform polarizing magnetic field.
 59. The process of claim 58,wherein the multi-echo NMR imaging sequence includes a plurality ofdifferent echoes, and wherein each of the plurality of different echoesis encoded differently.
 60. The process of claim 58, wherein themulti-echo NMR imaging sequence includes a plurality of differentechoes, and wherein at least one of the plurality of different echoes isencoded differently than another one of the plurality of differentechoes.
 61. The precess of claim 58, wherein the imaging object is ahuman being, and the uniform polarizing magnetic field is produced by amagnetic resonance imaging system, wherein the human being standsupright within the uniform polarizing magnetic field.
 62. An NMR imagingprocess, comprising: subjecting imaging object to a uniform polarizingmagnetic field; applying magnetic field gradients to the imaging object;applying RF energy to the imaging object according to a last-spin echotechnique; and subsequently applying RF energy to the imaging abjectaccording to a driven equilibrium technique; wherein the fast-spin echotechnique includes application of a multi-echo NMR imaging sequence;wherein the multi-echo NMR imaging sequence includes a first 90-degreeRF pulse followed by a series of 180-degree RF pulses; and wherein theseries of 180-degree RF pulses includes n 180-degree pulses, which arefollowed by n echoes.
 63. The process of claim 62, further comprisingapplying a second 90-degree RF pulse at a center of the nth echo, suchthat magnetization of the imaging object is oriented in the direction ofthe uniform polarizing magnetic field.
 64. The processes of claim 62,wherein the multi-echo NMR imaging sequence includes a plurality ofdifferent echoes, and wherein each of the plurality of different echoesis encoded differently.
 65. The process of claim 62, wherein themulti-echo NMR imaging sequence includes a plurality of differentechoes, and wherein at least one of the plurality of different echoes isencoded differently than another one of the plurality of differentechoes.
 66. The process as of claim 62, wherein the imaging object is ahuman being, and the uniform polarizing magnetic field is produced by amagnetic resonance imaging system, wherein the human being standsupright within the uniform polarizing magnetic field.
 67. An NMR imagingprocess, comprising: subjecting the imaging object to a uniformpolarizing magnetic field; applying magnetic field gradients to theimaging object; applying RF energy to the imaging object according to afast-spin echo technique, including applying an RF pulse correspondingto the angular precession frequency for a selected plane of the imagingobject; subsequently applying RF energy to the imagine object accordingto a driven equilibrium technique; detecting a nuclear magneticresonance signal emitted by the imaging object; processing the nuclearmagnetic resonance signal to provideimaging data; and after providingthe imaging data, moving the imaging object and applying an RF pulsecorresponding to the same angular precession frequency, to select adifferent plane of the imaging object.
 68. The process of claim 67,wherein the imaging object is a human being, and the uniform polarizingmagnetic field is produced by a magnetic resonance imaging system,wherein the human being stands upright within the uniform polarizingmagnetic field.
 69. An NMR imaging process, comprising: subjecting theimaging object to a uniform polarizing magnetic field; applying magneticfield gradients to the imaging object; applying RF energy to the imagingobject according to a fast-spin echo technique, including applying an RFpulse corresponding to the angular precession frequency for a selectedplane of the imaging object; subsequently applying RF energy to theimaging object according to a driven equilibrium technique; detecting anuclear magnetic resonance signal emitted by the imaging object;processing the nuclear magnetic resonance signal to provide imagingdata; and after providing the imaging data, applying an RF pulsecorresponding to a different angular precession frequency, to select arespective different plane
 70. The process of claim 69, wherein theimaging object is a human being, and the uniform polarizing magneticfield is produced by a magnetic resonance imaging system, wherein thehuman being stands upright within the uniform polarizing magnetic field.